A suspension for a tracked vehicle includes a bogie wheel assembly which has a pair of bogie wheels mounted to a walking beam. A number of beads are slidably positioned on the rod. The pattern-amount pairs are accumulated into a single finite-state machine, with each state having a list of pairs instead of a single value. Preferably the invention is used in a riveting tool for setting self-piercing rivets; as an alternative it may be used in a clinching tool. By arranging a zooming group having the positive third and negative fourth lens groups without arranging a positive fixed lens group in back of the prism, the overall length can be easily reduced as compared to the structure arranging a fixed lens group between a prism and a zooming group.